1. Field of the Invention
The present invention relates to an optical transmission line for transmitting signal light, and to an optical communication system including the same.
2. Description of the Related Art
The optical communication system is used for transmitting information by transmitting signal light through an optical transmission line, and is capable of transmitting a large capacity of information. When transmitting signal light through the optical transmission line, it is important to suppress the degradation of a signal. From this viewpoint, it is desirable that the overall dispersion of an optical transmission line has a small absolute value even in the case of a long transmission distance. It is also desirable that the optical transmission line be subject to less non-linear optical phenomena.
In general, a standard single-mode optical fiber used as an optical transmission line has a zero dispersion wavelength in the vicinity of a wavelength of 1.3 μm, and has a chromatic dispersion of about +17 ps/nm/km with a dispersion slope thereof being positive, in the vicinity of a wavelength of 1.55 μm, which is the signal light wavelength thereof. Therefore, forming the optical transmission line in a relay section using single-mode optical fibers alone would not allow the optical transmission line to have a small absolute value of cumulative dispersion for a long transmission distance. Under such circumstances, various proposals have hitherto been made for optical transmission lines having a small absolute value of the cumulative dispersion even in the case of a long transmission distance.
For example, Japanese Unexamined Patent Application Publication No. 11-355206, and U.S. Pat. Nos. 5,778,128, 5,611,016, and 6,178,279 each disclose an optical transmission line (related art 1) in which a positive-dispersion optical fiber and a negative-dispersion optical fiber are connected in series or they are alternately connected.
U.S. Pat. No. 6,084,993 discloses an optical transmission line (related art 2) that has a configuration in which a non-linearity diminishing optical fiber with a large effective area, a dispersion adjusted transmission optical fiber with a small negative chromatic dispersion, and an optical fiber for diminishing and adjusting a dispersion slope, which is used to adjust the overall dispersion slope, are connected in this order.
U.S. Pat. No. 5,995,694 discloses an optical transmission line (related art 3) that has a configuration in which a standard single-mode optical fiber, a dispersion compensation optical fiber, of which the chromatic dispersion and the dispersion slope are both negative, and a dispersion flat optical fiber with a dispersion slope of a small absolute value, are connected in this order.
Also, U.S. Pat. No. 5,894,537 discloses an optical transmission line (related art 4), which is a so-called “dispersion managed optical fiber”. The dispersion managed optical fiber is formed such that positive dispersion regions each having a positive chromatic dispersion, and negative dispersion regions each having a negative chromatic dispersion are alternately disposed along the longitudinal direction thereof.